It has previously been proposed to control the energy supply, that is, electrical energy, to a glow plug for use in a Diesel engine, especially an automotive-type Diesel engine. It is possible to provide a simulated circuit which simulates the temperature-resistance curve or the temperature-voltage supply curve of operation of the glow plug, in order to prevent overheating of the glow plug and damage thereto under various environmental conditions, including excess voltage supply conditions, for example. The simulating circuit then is so arranged that it controls supply and disconnection of energy to the glow plug in accordance with the temperature characteristics thereof, that is, permits connection of electrical power to the glow plug only to the extent which is necessary to insure optimum operation of the glow plug. The ON and OFF switching conditions, within respective switching cycles, can be controlled to define predetermined upper and lower temperatures which the glow plug may assume, that is, when the glow plug reaches an upper temperature, energy supply is turned OFF, to be again turned ON when the temperature of the glow plug has dropped below a lower limit.
Circuits which simulate the temperature characteristics of a glow plug are complex. It is, therefore, difficult to form an analog simulation of the glow plug at reasonable expense. Further, it is necessary to calibrate the circuit before it is used. In spite of calibration, and setting of the circuit to match various conditions, it has been found that the control of the switching cycle is imprecise. Optimal control of the temperature of the glow plug, by use of simulation circuitry, or mathematical models which are stored in circuitry, thus is not readily possible.